In telecommunications systems for civilian use there is a problem of providing microwave filters allowing the various transmission channels to be allocated in the desired frequency bands. Usually these filters are implemented with a plurality of cavity resonators mutually coupled through irises, screws or the like.
When such filters are to be used in transponders installed on board a satellite, the resonator size has to be as small as possible. In fact, since some ten filters could be used and each filter is generally composed of 4 to 8 resonators, the encumbrance is considerable. Namely, at a center frequency of 12 GHz, a 6pole filter implemented with dual-mode cylindrical cavities has, as a whole, a 30 mm diameter and a 60 mm length.
Recently it has been suggested to introduce a small dielectric cylinder into each cavity resonator to reduce the filter sizes. This has been rendered possible by the availability of high-permittivity, low-loss, high temperature-stability dielectric materials.
The high permittivity of the material introduced into the resonator ensures that the electromagnetic field will be practically completely concentrated inside it. That is why the cavity dimensions, calculated to obtain the resonance at a determined wavelength, are greatly reduced. Under the same conditions as those of the preceding example, the total dimensions of an equivalent filter with dielectric-loaded resonators decrease to about 20 mm for the diameter and 30 mm for the length, with an overall reduction to less than a fourth of the original volume.
One of the problems encountered in implementing a dielectric-loaded resonator of this kind is to conveniently support the small dielectric cylinder placed inside the resonator. In fact dielectric material cannot completely fill up the metallic cavity both because of the high loss increase due to the contact between metal and dielectric and of the necessity of inserting tuning screws into the lateral resonator surface. Hence, it is required to provide a supporting structure for the dielectric material, which is capable of holding it in the correct position without detriment to its electrical characteristics, by keeping losses low, and of assuring the necessary mechanical stability of the structure, chiefly for use on board a satellite.
The article entitled "Dielectric-Resonators Design Shrinks Satellite Filters and Resonators" by S. Jerry Fiedziuszko, in MSN & CT, August 1985, describes a cylindrical cavity resonator of the same type as those conventionally used in unloaded filters, into which an ultra-low-loss ceramic material cylinder is introduced. The small dielectric cylinder is held in correct position by a plastic material disk or by a more complex support made of silicon foam.
Yet this solution presents a number of inconvenience if the filter is to be used for processing signals even with moderate powers. In fact plastic material can tolerate moderate temperatures, usually lower that 100.degree. C., and silica foam has extremely low thermal conductivity; that is why the heat produced in the dielectric cylinder is only partly dissipated.
In addition, by using a single supporting disk, as can be seen from FIG. 11 of the cited article, mechanical stability seems rather limited, unless adhesives are used between the disk and the small dielectric cylinder, which considerably increase losses.
Other solutions providing the use of supporting disks made of different materials, such as alumina or forsterite, are not considered satisfactory by the author of the above-mentioned article owing to their poor temperature stability.